Desalination is the process of removing dissolved salts and minerals from seawater or brackish water to produce fresh, drinkable water, known as potable water. This technology is becoming increasingly significant as a solution to global water scarcity, driven by population growth and changing climate patterns that stress traditional freshwater supplies. Over 70% of the Earth is covered in water, but only a tiny fraction is readily accessible freshwater. Desalination plants worldwide already produce tens of millions of cubic meters of water daily, illustrating its growing role in water security strategies.
Heat-Based Desalination: Simple and Industrial Distillation
The most intuitive method for separating salt from water is distillation, which mimics the natural water cycle. When salt water is heated, the water molecules vaporize into steam, leaving all the dissolved salts and impurities behind. The pure steam is then collected and condensed back into liquid freshwater. This simple principle is used in small-scale applications like solar stills, which capture solar energy to evaporate a small amount of water for survival purposes.
Industrial-scale thermal desalination uses sophisticated processes to improve efficiency, such as Multi-Stage Flash (MSF) distillation and Multi-Effect Distillation (MED). MSF involves heating seawater and then introducing it into a series of chambers, each maintained at a successively lower pressure. The sudden drop in pressure causes a portion of the water to instantly “flash” into steam, which is then condensed. MED uses a sequence of evaporators, known as effects, where the steam produced in one effect is used to heat the seawater in the next, thereby reusing the thermal energy.
These thermal methods can produce extremely high-purity water. However, they are inherently energy-intensive, typically requiring 80 to 120 kilowatt-hours of thermal energy per cubic meter of water. This high heat requirement makes them more prone to scaling and corrosion, although newer designs like MED operate at lower temperatures, around 70°C, to mitigate these issues.
Pressure-Based Desalination: The Role of Reverse Osmosis
The dominant modern method for large-scale seawater desalination is Reverse Osmosis (RO), which relies on mechanical pressure rather than heat. The process begins with osmosis, where water naturally flows across a semi-permeable membrane from a region of low salt concentration to a region of high salt concentration. The pressure needed to stop this natural flow is called osmotic pressure.
Reverse Osmosis works by overcoming this natural osmotic pressure through the application of extreme mechanical pressure to the saltwater side. This applied pressure forces the water molecules through the microscopic pores of the semi-permeable membrane, against the natural gradient. The membrane is engineered to allow only the small water molecules to pass through, while blocking the larger dissolved salt ions, which are typically rejected at a rate of up to 99%.
The high-pressure pumping system is a central component of an RO plant. RO is the preferred technology today because it is significantly more energy-efficient than thermal distillation, generally requiring only about 2 to 4 kilowatt-hours of electrical energy per cubic meter of desalinated water. This lower energy demand, coupled with the modularity of membrane systems, has made RO the most widely deployed technology globally. The purified water, called permeate, is collected on one side, while the highly concentrated salt solution, or brine, is collected on the other.
The Practical Challenges: Energy Use and Brine Management
Desalination remains an energy-intensive process compared to using conventional freshwater sources. The total energy consumption for seawater RO is substantial, with a theoretical minimum energy requirement around 1 kilowatt-hour per cubic meter of water produced. To offset the energy cost of the high-pressure pumps, modern plants often incorporate energy recovery devices that recapture hydraulic energy from the rejected, high-pressure brine stream to pre-pressurize the incoming seawater.
A major challenge is the management of the brine, which is the concentrated waste stream from the desalination process. For every cubic meter of freshwater produced, a significant volume of hypersaline effluent is generated, containing salt concentrations up to twice that of the original seawater. Most plants dispose of this brine by discharging it back into the ocean through outfalls.
This discharge poses an environmental risk because the brine is often warmer and much saltier than the surrounding seawater, sometimes containing residual pretreatment chemicals. When released without adequate mixing, the dense, hypersaline plume can settle on the seabed, negatively impacting local marine ecosystems, particularly sensitive organisms like seagrass meadows. To mitigate this impact, plants use diffusers and submerged outfalls that are engineered to maximize the rapid mixing and dilution of the brine with the ambient seawater.